The Republican presidential debates have mattered more this year than anything else in determining which candidate had the momentum and the lead in the race for the White House nomination.

Thus, Thursday evening's Sioux City, Iowa debate (Fox News, 9 pm ET) could be decisive in narrowing the gap between Newt Gingrich and the rest of the field or cementing his frontrunner status.

The debate is the last one before the Jan. 3 Iowa caucuses so the pressure will be intense on the candidates to leave the most positive impression possible on Iowans as voters' attention will soon turn to their last-minute preparations for the holidays.

For Gingrich, that means continuing his string of strong debate performances.

With the rhetorical ability to "float like a butterfly and sting like a bee" to use Muhammad Ali's old line, Gingrich has shown the ability to sidestep potentially dangerous debate moments.

For instance, during the Des Moines debate, Rep. Ron Paul of Texas attacked the former speaker for taking money in his post-Congress years from mortgage giant Freddie Mac. Gingrich responded by saying he agreed with Paul that the Federal Reserve caused the 2007-2008 economic crisis.

It was part of Gingrich's strategy to not go negative, which he has mostly kept to as he tries to remake his 1990s reputation as a politician with an unseemly appetite for the bloodsport aspect of modern political combat.

Given his stated intention, we can expect Gingrich to try to stay above the fray.

Mitt Romney will likely try to make it difficult for Gingrich to do that. The former Massachusetts governor has been going negative on Gingrich, using the word "zany" in a New York Times interview Wednesday to make the point that whoever GOP voters choose as their White House nominee will need to have "sobriety."

He didn't say Gingrich was unstable, of course; he only implied it. Romney's obvious point: he, not Gingrich, had the stability to have codes to the nation's nuclear arsenal at his disposal.

But while Romney will be trying to get under Gingrich's skin to get the former speaker to stumble, he'll need to be on guard for Gingrich and others doing the same to him.

Texas Gov. Rick Perry especially has seemed to have great success pushing Romney's buttons, getting him to make some of the most memorable gaffes of the debates.

The most recent happened in Des Moines just days ago when Romney made his ill advised $10,000 bet offer in an attempt to out maneuver Perry on a point about Romney's stand on the individual healthcare insurance mandate.

And at an earlier debate, it was Perry who attacked Romney for having hired a lawncare service which repeatedly used illegal immigrants on Romney's property, causing Romney to say that he eventually told the contractor to not bring the undocumented workers because "I'm running for office, for Pete's sakes."

The other problem for Romney, of course, is that attacking Gingrich at the debate won't do much to help Romney with his likeability problem and may even worsen it with some Iowa voters.

Paul, who has been making a strong showing in Iowa polls, in the top three in a number of them, will likely help Romney make the anti-Gingrich case.

Paul seems to have a visceral dislike for Gingrich. He was first out with the harshest ads against Gingrich, famously accusing him in one of "serial hypocrisy." There's no reason to expect he would relent Thursday evening.

Three of the four remaining candidates — Perry, Rep. Michele Bachmann of Minnesota and Rick Santorum, the former U.S. senator from Pennsylvania, have each made major plays for the caucus votes of social conservatives in Iowa.

They appear to be splitting that vote. So you'd expect each of them to aim their appeals Thursday evening at trying to consolidate that voting bloc behind his or her candidacy while continuing their argument that neither Gingrich or Romney is truly conservative enough to be the Republican standard bearer.

Indeed, Bachmann made that argument at the recent Des Moines debate, creating a hybrid candidate she was running against she called "Newt Romney."

But that wasn't enough to change the shape of the race.

Jon Huntsman Jr., the former Utah governor, wrote off Iowa some time ago. So his appeal will directed less at Iowans than New Hampsherites since he hopes to have a strong enough showing in the Granite State to get Republican voters to reconsider his candidacy.

It's a long shot strategy but he doesn't have many other options at this point.

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